Method for differentially heating a screening surface



June 1961 E. M. BURSTLEIN 2,989,182

METHOD FOR DIFFERENTIALLY HEATING A SCREENING SURFACE Filed March 5, 1957 3 Sheets-Sheet 1 INVENTOR BY WM Co&,,JMf Maw ATTORNEYS June 20, 1961 METHOD FOR DIFFER Fild' March 5. 1957 E. M. BURSTLEIN 2,989,182

ENTIALLY HEATING A SCREENING SURFACE 3 Sheets-Sheet 2 4 d INVENTOR Zzgezee 1131073276075 BYWom, MMdMaW ATTORNEYS June 20, 1961 E. M. BURSTLEIN METHOD FOR DIFFERENTIALLY HEATING A SCREENING SURFACE Filed March 5. 1957 3 Sheejzs-Sheet 3 IINVENTOR Q Ezge/z e/ZBzwzsZkzk B E/dam, Co,J/w'nd. 4 Q5130 ATTORNEYS United States In screening apparatus, it is well known that a slight heating of the screening gauze greatly facilitates the screening of moist materials and prevents a rapid clogging of the screen by the finer particles coating the wires of the gauze.

One convenient way of effecting such heating is by electric Joule effect, the screening gauze being used as the heating resistance. In conventional methods of this type single-phase current is generally used, or sometimes three-phase current may be employed as described in the U.S. patent application Serial No. 388,006, filed October 23, 1953, now Patent No. 2,868,378.

Where the screening surface is supplied with singlephase current, one of the three phases of the power supply network is usually converted into low or very lowvoltage single-phase current, and this has the disadvantage of introducing an unbalance into the network.

Where the screen is fed with three-phase current the three phases of the supply network power are generally converted into lowto very low-voltage three-phase current, thereby avoiding the said unbalance in the network.

In this latter case however the screening surface has to be subdivided into a number of panels equal to three or a multiple of three. This may be an inconvenience in connection with comparatively low-capacity screens wherein the screening area is not sufficiently large to warrant the increased complications in the construction of the screen and the wiring involved in supplying the three phases to the apparatus and feeding of the three currents into the screening web.

It is an object of my invention to provide a method of heating a screening surface overcoming these drawbacks.

Another object of my invention is to provide a method of heating a screening surface by Joule effect from a twophase electric source, the screening surface being divided into an even number of panels one half of which are fed with current of phase I and the other half with current of phase II.

Another object of my invention is to provide a method of heating a screening surface as specified in the preceding paragraph wherein the heating energy is derived from a three-phase electric network without unbalancing said network, by providing between said three-phase network and the screening surface to be heated a 3-to-2 phase conventing transformer whose secondary windings are connected respectively to one half of the panels in which the screening surface is divided.

Another object of my invention is to provide a method of heating a screening surface allowing a simpler and hence cheaper construction of the screen than if a threephase supply were used, while the resulting screens will nevertheless display all the advantages obtained with three-phase current heating.

Yet another object of my invention is to provide a method of heating a screening surface by dividing said surface in an even number of panels fed from a three atent phase electric network through the intermediary of a 3-to-2 phase converting transformer permitting: (a) a the unused surface of the screen, (c) the employment of Patented June 20, 1961 the differential type of heating described in the above specified US. patent application thereby affording a saving of up to about 30% in power consumption.

Preferably the 3-to-2 phase converting transformer delivers on its secondary side an adjustable two-phase voltage such that the maximum R.M.S. voltage will not exceed 24 volts per phase, so that the heating of the screening surface may be matched with the character of the materials to be sifted, the degree of humidity thereof, the mesh size of the screen and the resistivity of the metal from which the screening surface is made. The transformer used may be of any suitable form but preferably it allows voltage variations across the secondary windings according to a series of (uniformly or non uniformly) stepped values with the highest value not exceeding e.g. 24 volts.

Such adjustment of the secondary voltage is obtained by switching the connections of the three primary windings which may be coupled in Y- or in delta-relationship. Furthermore each said winding is provided with additional taps for deriving intermediate voltage values so that the output voltages obtainable by such switching may be varied by uniform increments.

In addition, the number of ampere-turns of the secondary windings are so selected that the two output currents may be delivered at a common voltage or at different voltages.

It is thus possible, by dividing the screening surface into two or more consecutive panels in the direction of flow of the materials, to achieve a differential heating of such panels by imparting thereto similar electrical character istics (impedance) and applying different voltages thereto, or alternatively imparting different characteristics (impedance) to the panels while applying equal voltages thereto. The two methods may likewise be combined.

In order to achieve a balanced differential heating, it is possible as described in the above mentioned US. patent application in connection with three-phase heater systems, to impart to the panels or sets of panels different impedance values per unit area and so to modify the relative lengths thereof that the total impedances of both panels or sets of panels will be equal.

It should be noted that since the two output phases of the transformer are utilized independently of each other for heating two, or an even number of, panels, only the R.M.S. voltage of each output current has to be taken into account, so that the relative phase displacement between the output currents may be different from The following description with reference to the accompanying drawings given by way of example but in no way with a limitative sense will provide a clear understanding of the manner in which the invention may be performed, it being understood that the features standing out both from the drawings and the description form a part of this invention.

FIGS. 1 to 6 illustrate various methods of dividing a screen into panels each adapted to be heated in accordance with the invention;

FIG. 7 is a plan view of means for attaching a panel to a screen stretching or anchoring bar and for feeding current to the panel;

FIG. 8 is a vertical section on an enlarged scale of the arrangement shown in FIG. 7;

FIG. 9 is a sectional view similar to that of FIG. 8 but illustrating a modified construction; and

FIG. 10 is a large scale vertical section of electrical connector means between a pair of panels insulated from the screen casing or frame.

In the simplest method of dividing a screening surface and feeding two-phase current thereto, there are provided two panels A and B (FIG. 1) mounted in a casing or on frames not shown, and connected to receive electric current through their extremities e.g. by the means to be described hereinafter.

Phase I of the secondary of a 3-to-2 phase converter transformer is fed to panel A over transversely extending busbars 1 and 2. Phase II from said transformer is fed to panel B over busbars 2 and 3.

This type of division and heating of the screening surface, more particularly applicable to medium-size screening plants has the following advantages: Screen construction remains very easy, and no sudden variation in elec tric voltage occurs between any two adjacent points of the screening surface.

The amounts of power supplied to each panel per unit area may differ from each other so as to achieve differential heating as described in the patent mentioned above, and the second panel B has a length so predetermined that the total energy received by it will equal that supplied to the first panel A in order to ensure balanced differential heating. This makes it possible to load equally both secondary windings of the transformer thereby reducing the losses in the transformer.

Information relating to both types of heating just mentioned may be found in the U.S. patent mentioned above.

If a wide and comparatively short screen is required, the screening surface may be divided as shown diagrammatically in FIG. 2 into two parallel longitudinal panels interconnected widthwise by an insulating joint or seal of suitable conventional type. The phase I current from the secondary is fed by way of busbars 4 and 5 to the opposite ends of panel C, and phase II current is fed through busbars 6 and 7 to the ends of panel D.

This construction is advantageous in that it leads to an inexpensive type of structure in the case of wide short screens.

The form of screen construction shown in FIG. 3 is particularly applicable to large-surface screens. The screening surface is divided into four panels E, F, G, H. Panels E and G, and panels F and H, form two pairs of surfaces respectively similar to the screen of FIG. 1, and both component surfaces are longitudinally connected by an insulating joint as in FIG. 2.

The phase I current supplied by the converter transformer secondary is fed through busbars 8, 9 and 10, 11, in parallel to both panels E and F, while phase II current is in a similar way fed over busbars 9, 12 and 11, 13 to panels G and H in parallel.

This construction includes the same advantages as those indicated for the constructions of FIGS. 1 and 2 from which it derives.

If the screen used includes two stages, or the screening plant includes two substantially similar screens, it may be desirable to use the set-up shown in FIG. 4. Each stage or each screen comprises two panels, altogether providing the four panels I, 'J, and K, L. Panels I and I are interconnected by a connecting bar 14 and panels K, L are interconnected by a connecting bar 15 so as to be mounted in series in each pair. The phase I current is fed over bars 16, 16a to both panels I and I which are mechanically interassembled by an insulating joint, while the phase II current is fed through bars 17, 17a to both panels K and L also mechanically assembled by an insulating joint.

The arrangement shown in FIG. 5 is advantageous in the case of two single-stage screens arranged in tandem relation. The screening surface of the first or upstream screen is divided into two panels M and N. The phase I current from the converter transformer is fed to panels M and N through bars 18, 19 and 20, 21. The screening surface of the second or downstream screen is also divided into two panels O and P and the phase II current from the supply transformer is fed into said panels over terminals 22, 23 and 24, 25.

This form of embodiment has the fol-lowing advantages: Greatly simplified construction of each of the two screen units; absence of any voltage discontinuity between adjacent points of the screening surface; use of balanced differential heating is rendered extremely easy, it being effected either by using different voltages for the two secondary circuits of the converter transformer, or by providing different characteristics for the panels M and N of the first screen and for the panels 0 and P of the second screen unit.

In the case of screens having large screening areas, and where it is impossible or impractical for any reason to feed current to the centre of the casing, the set-up schematically illustrated in FIG. 6 may be used. The total screening area is in this case subdivided into four panels Q, R, and S, T electrically connected in series, in each set of two panels, by means of a cross connector bar 27 and 28. These cross connector bars extend transversely substantially at the midpoint of the length of the screens, are parallel and are mechanically connected together by an insulating joint not illustrated, of any suitable type. The free ends of the panels Q and R are connected by bars 29 and 29a to the supply transformer secondary terminals supplying the phase I current, while the free ends of panels S and T are connected by bars 30' and 30a to the transformer secondary terminals supplying the phase II current.

This form of embodiment exhibits the following advantages: Absence of any electric supply terminal in the central part of the screen apparatus; balanced differential heating can be applied; the magnitude of the electric currents required is only half that required in the form of embodiment of FIG. 5.

Should it prove impossible or impractical for any reason to feed current to the screening gauze through the ends of the screen casing, an arrangement may be used similar to FIG. 6 but wherein the relative positions of the cross connector bars 27 and 28 and the busbars 29, 29a and 30, 30a connected to the transformer are reversed, the former being connected to the ends of the screen and the latter to the central part thereof.

In some types of screens the screening elements are stretched across removable frames which frequently are arranged in superimposed stages. In such cases it will be especially desirable to use the arrangement shown in FIG. 4. The panels I and J are secured on one frame and panels K and L on another frame, each set of panels being fed with one of the phase currents I and II in the manner described with reference to FIG. 4. The panels I and J are connected in series, and so are the panels K and L, so that this set-up similar to that of FIG. 5 has the advantage of halving the currents required for heating a given screen area.

In feeding heating current to conventional screen elements, with a magnitude of such current under very low voltage sometimes attaining several thousands of amperes in conventional screening plants having several square meters of useful screening area, it was heretofore customary to provide the electrical connection between the supply end of the screen gauze and the busbar in the form of U-shaped connectors. The end of the screening gauze was inserted between the legs of the U and held therein by means of soft metal cast in the connector. It has been found that such a connector is not indispensable and that the low-voltage current fed to the screening gauze may conveniently be fed without excessive electric losses by a direct contact of the gauze with the busbar provided such contact be effected and maintained with a firm and uniform pressure throughout the length of the connection.

FIGS. 7 and 8 illustrate at different scales a form of construction of such a connection free of soldered terminal connectors, and applicable specifically to an end of the screen. A plate 43 forming part of the means used for mechanically stretching the gauze and compensating for the expansion thereof consequent on heating, is slidab ly mounted at each end over a guide secured to the screen frame or casing, by way of an interposed electrical insulator. Threaded rods 44 arranged in two parallel rows and in staggered relation are secured as by screwing and/or solder to the plate 43 at small equispaced intervals and project through the busbar 45 preferably made from pure copper. The end part of the screening element 46 to be connected has holes formed in it for the threaded rods 44 to pass therethrough, and is engaged over said rods and pressed into contact engagement with busbar 45. A backing or pressure plate 47 is then applied over the screen gauze element and the entire assembly firmly clamped with nuts 41 whereupon the nuts are blocked in tightly screwed condition by means of arrestors 42 having ends adapted to be bent back against the sides of adjacent nuts 41.

The securing means described have proved highly successful provided care is taken thoroughly to clean and degrease the mating areas to be electrically connected, namely the end portion of gauze element 46 and busbar 45. The resulting connection and seal is found to stand up in a perfectly satisfactory manner to the vibrational stresses imposed in service to the screen.

FIG. 9 illustrates in section a form of means for securing the screen element and connecting it with the busbar, especially applicable to small-size screens. This modification differs from that of FIG. 8 only in that the two ranks of threaded rods are replaced by a single line of bolts '50 having their heads soldered to the bottom end of plate 43.

The electrical connection between adjacent panels may be provided in the form shown in FIG. 10. Secured to a part rigid with the screen finame, such as the crossbar 51, is an intermediate plate 52 insulated from the crossbar by an insulating sheet insert 53, and insulating bushings 54 mounted on the threaded rods 55 where the rods extend through the crossbar 51. The downwardly extending threaded rods 55 are disposed in two parallel rows, are secured by threaded connections and/or solder to the plate 52 so as to maintain the latter against the crossbar 51 by means of nuts 56 and clamping washers 57. Two further rows of screw rods 58 are threaded and soldered into the intermediate plate 52 secured to and insulated from the screen frame as just described, said further rods 58 projecting above the plate. One of the screening elements 46 to be electrically connected to the other element 59 through a cross-connector bar 45 is engaged over the screw rods 58 and is pressed into contact with plate 52, and thereafter the cross connector bar 45 is placed over the gauze 46, the other gauze element 59 is then brought into engagement with the bar 45 and the entire assembly is covered up with the overlying backing plate 47 against which the clamping nuts 41 are screwed down and blocked with arrestors 42. The re sulting assembly is generally similar to that of FIG. 8, except that one screening element is firmly clamped against each of the upper and lower surfiaces of the cross connector bar 45, preferably made of copper.

Moreover, the screening gauze may be divided in other ways than those described by way of example, provided the screen be suitably divided into an even number of panels.

What I claim is:

A method for differentially heating by Joules effect a screening surface divided in an even number of electrical conductive panels, from a three-phase electric network without substantially unbalancing said network, which comprises electrically connecting said panels in a pair of groups of panels each including the same number of panels, electrically insulating each group from the other, converting the three-phase current provided by said network into a two-phase current, adjusting the voltage of each phase of said two-phase current independently from the voltage of the other phase of said two-phase current, feeding the first group of panels with the first phase of said two-phase current, feeding the second group of panels with the second phase of said two-phase current, and adjusting the ratio of the voltages of the phases of said two-phase current for feeding substantially to said group of panels located in the upstream position relative to the introduction of particulate material thereto, a quantity of energy per unit area greater than the quantity of energy fed per unit area to the second group of panels.

References Cited in the file of this patent UNITED STATES PATENTS 344,021 .Vickery June 22, 1886 1,710,795 Arms Apr. 30, 1929 2,005,912 Drake June 25, 1935 2,255,939 Overstrom et \al. Sept. 16, 1941 2,314,879 Haller Mar. 30, 1943 2,704,155 Hannon Mar. 15, 1955 2,826,304 Stone et a1 Mar. 11, 1958 2,868,378 Burstlein Jan. 13, 1959 FOREIGN PATENTS 247,791 Great Britain Feb. 25, 1926 665,313 Great Britain Jan. 23, 1952 682,028 Great Britain Nov. 5, 1952 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2.389,, 182 June 20 1961 Eugene M Burstlein It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the grant lines. 2 and 3 for ""assignor to Societe des Acieries de Longwy (Societe Anonyme) read assignor of one-half to Societe des Acieries de Longwy (Societe Anonyme) lines, 12 and l3 for "Societe des Acieries de Longwy (Societe Anonyme) v its successors" read Eugene Ma Burstleim his heirs or assigns, and Societe des Acieries de Longwy (Soci ete Anonyme) Y its successors ---g and in the heading to the printed specification, lines, 4 to 6 for 'assignor to Societe des Acieries de Longwy (Societe Anonyme)" read assignor of one-half to Societe des Acieries de Longwy (Societe Anonyme) Signed and sealed this 10th day of April 1962,

(SEAL) Attest:

ERNEST Wu SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

